This invention relates to an ultrasonic transducer on medical apparatus that is introduced into the body and especially to a method of characterizing arterial disease by generating ultrasound from within the artery.
Atherosclerosis, hardening of the arteries, afflicts most people living in developed countries. Coronary artery disease is the primary cause of ischemic heart disease, which is responsible for more deaths in this country than any other disease. Treatment of artherosclerosis currently relies on considerable guess work by the physician. Some forms of plaque, which are calcified lesions on arteries, may be effectively treated by drugs while others require surgery. Also, approximately 30% of patients who undergo balloon angioplasty for opening of occluded arteries, the major cause of heart attacks, redevelop the occlusion within three months. In order to select the proper course of treatment and to assess the success of treatment, some means to differentiate types of plaques is needed.
Most work attempting to characterize artery disease has used ultrasound at the relatively low frequencies of 5 to 15 MHz. This is because the transducers were placed on the surface of the skin, and therefore the ultrasound has to penetrate considerable distance before the artery was reached. Because of the high acoustic attenuation at the higher frequencies of 25 to 50 MHz, these frequencies could not be used to penetrate deep enough into the body to characterize arteries. Even carotid arteries which lie very close to the skin in the neck are typically examined with 15 MHz ultrasound. Due to the long wavelengths and broad beamwidths of these systems little success has been achieved in differentiating various types of atherosclerotic plaques even in relatively accessible arteries such as the carotid. This work is described at length in the medical literature.
Broadband high frequency (25 to 50 MHz) focused transducers have been available for many years, and an ultrasonic microscope represents one of the most significant applications of these transducers to nondestructive testing. Refer for instance to "Acoustic Microscopy From 10 to 100 MHz for Industrial Applications", R. S. Gilmore et al., Phil. Trans. R. Soc. Lond. A320, 215-235 (1986). These systems are inappropriate for introduction into arteries because of the size of the transducers, typically several inches. Measurements in such an ultrasonic microscope on excised artery specimens using a broadband 50 MHz transducer focused with an F/2 lens indicate that the system can differentiate between normal artery and fatty plaques, an early stage of atherosclerosis not normally detectable.
The ultrasonic transducers of this invention may be fabricated from thin piezoelectric polymer film such as PVDF (poly vinylidene di fluoride). High frequence transducing elements of this material are described in patent 4,760,304, D. W. Oliver, "Dark Field Coaxial Ultrasonic Transducer" and allowed divisional application S.N. 126,325, filed Nov. 30, 1987, "Method of Fabricating Dark Field Coaxial Ultrasonic Transducer" now U.S. Pat. No. 4,787,126.